Here's why:
* Thermal energy is the energy associated with the random motion of atoms and molecules within a substance.
* Gases have the weakest intermolecular forces between their molecules. This means the molecules are free to move around and vibrate with greater energy.
* Liquids have stronger intermolecular forces than gases, so their molecules have less freedom of movement and therefore less thermal energy.
* Solids have the strongest intermolecular forces, meaning their molecules are tightly packed and have the least freedom of movement, resulting in the lowest thermal energy.
In summary, the higher the thermal energy, the more freedom of movement the molecules have, and this is most evident in the gaseous phase.
